Variable strength stent

ABSTRACT

An expandable stent for implantation in a body lumen, such as an artery, is disclosed. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common longitudinal stent axis and interconnected by one or more interconnecting members placed so that the stent is flexible in the longitudinal direction. The strength of the stent at a center section or at either end can be varied by increasing the mass of the struts forming each cylindrical element in that center section or end section relative to the lower mass struts in the remaining sections of the stent. Increasing the mass of the struts can be accomplished by, for a given strut thickness, increasing the width of the strut, or increasing the length of a cylindrical element.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to expandable endoprosthesisdevices, generally known as stents, which are designed for implantationin a patient's body lumen, such as blood vessels to maintain the patencythereof. These devices are particularly useful in the treatment andrepair of blood vessels after a stenosis has been compressed bypercutaneous transluminal coronary angioplasty (PTCA), percutaneoustransluminal angioplasty (PTA), or removed by atherectomy or othermeans.

[0002] Stents are generally cylindrically-shaped devices which functionto hold open and sometimes expand a segment of a blood vessel or otherlumen such as a coronary artery. They are particularly suitable for useto support the lumen or hold back a dissected arterial lining which canocclude the fluid passageway therethrough.

[0003] A variety of devices are known in the art for use as stents andhave included coiled wires in a variety of patterns that are expandedafter being placed intraluminally on a balloon catheter; helically woundcoiled springs manufactured from an expandable heat sensitive metal; andself-expanding stents inserted in a compressed state and shaped in azigzag pattern. One of the difficulties encountered using prior artstents involved maintaining the radial rigidity needed to hold open abody lumen while at the same time maintaining the longitudinalflexibility of the stent to facilitate its delivery and accommodate theoften tortuous path of the body lumen.

[0004] Another problem area has been the limited range of expandability.Certain prior art stents expand only to a limited degree due to theuneven stresses created upon the stents during radial expansion. Thisnecessitates providing stents with a variety of diameters, thusincreasing the cost of manufacture. Additionally, having a stent with awider range of expandability allows the physician to redilate the stentif the original vessel size was miscalculated.

[0005] Another problem with the prior art stents has been contraction ofthe stent along its longitudinal axis upon radial expansion of thestent. This can cause placement problems within the artery duringexpansion.

[0006] Various means have been described to deliver and implant stents.One method frequently described for delivering a stent to a desiredintraluminal location includes mounting the expandable stent on anexpandable member, such as a balloon, provided on the distal end of anintravascular catheter, advancing the catheter to the desired locationwithin the patient's body lumen, inflating the balloon on the catheterto expand the stent into a permanent expanded condition and thendeflating the balloon and removing the catheter.

[0007] What has been needed is a stent which not only addresses theaforementioned problems, but also has variable strength, yet maintainsflexibility so that it can be readily advanced through tortuouspassageways and radially expanded over a wider range of diameters withminimal longitudinal contraction to accommodate a greater range ofvessel diameters, all with minimal longitudinal contraction. Certainly,the expanded stent must have adequate structural strength (hoopstrength) to hold open the body lumen in which it is expanded. Thecontrol of stent strength at specific locations along the stent resultsin a highly customizable device specifically adapted to the unique bodylumen formation in the patient.

[0008] One approach to the variable strength problem is to increasestrut thickness. This technique is disclosed in co-pending applicationSer. No. 08/943,992, filed Oct. 3, 1997, by T. Limon and T. Turnlund,entitled “Stent Having Varied Amounts Of Structural Strength Along ItsLength,” whose entire contents are hereby incorporated by reference.Another approach is to vary the length or width of the strut at aconstant strut thickness. The present invention is directed to thisapproach.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to stents of enhancedlongitudinal flexibility and configuration which permit the stents toexpand radially to accommodate a greater number of different diametervessels, both large and small, than heretofore was possible. The stentsof the instant application also have greater flexibility along theirlongitudinal axis to facilitate delivery through tortuous body lumens,but remain highly stable when expanded radially, to maintain the patencyof a body lumen such as an artery or other vessel when implantedtherein. The unique patterns of the stents of the instant inventionpermit both greater longitudinal flexibility and enhanced radialexpansibility and stability compared to prior art stents.

[0010] Each of the different embodiments of stents of the presentinvention includes a plurality of adjacent cylindrical elements whichare generally expandable in the radial direction and arranged inalignment along a longitudinal stent axis. The cylindrical elements areformed in a variety of serpentine wave patterns transverse to thelongitudinal axis and contain a plurality of alternating peaks andvalleys. At least one interconnecting member extends between adjacentcylindrical elements and connects them to one another. Theseinterconnecting members insure minimal longitudinal contraction duringradial expansion of the stent in the body vessel. The serpentinepatterns have varying degrees of curvature in the regions of the peaksand valleys and are adapted so that radial expansion of the cylindricalelements are generally uniform around their circumferences duringexpansion of the stents from their contracted conditions to theirexpanded conditions.

[0011] The resulting stent structures are a series of radiallyexpandable cylindrical elements that are spaced longitudinally closeenough so that small dissections in the wall of a body lumen may bepressed back into position against the lumenal wall, but not so close asto compromise the longitudinal flexibility of the stent both when beingnegotiated through the body lumens in their unexpanded state and whenexpanded into position. The serpentine patterns allow for an evenexpansion around the circumference by accounting for the relativedifferences in stress created by the radial expansion of the cylindricalelements. Each of the individual cylindrical elements may rotateslightly relative to their adjacent cylindrical elements withoutsignificant deformation, cumulatively providing stents which areflexible along their length and about their longitudinal axis, but whichare still very stable in the radial direction in order to resistcollapse after expansion.

[0012] Each of the stents of the present invention can be readilydelivered to the desired lumenal location by mounting it on anexpandable member, such as a balloon, of a delivery catheter and passingthe catheter-stent assembly through the body lumen to the implantationsite. A variety of means for securing the stents to the expandablemember of the catheter for delivery to the desired location arcavailable. It is presently preferred to compress or crimp the stent ontothe unexpanded balloon. Other means to secure the stent to the ballooninclude providing ridges or collars on the inflatable member to restrainlateral movement, using bioabsorbable temporary adhesives, or adding aretractable sheath to cover the stent during delivery through a bodylumen.

[0013] The presently preferred structures for the expandable cylindricalelements which form the stents of the present invention generally have acircumferential serpentine pattern containing a plurality of alternatingpeaks and valleys. The degrees of curvature along adjacent peaks andvalleys are designed to compensate for the stresses created duringexpansion of the stent so that expansion of each of the peaks andvalleys is uniform relative to one another. This particular structurepermits the stents to radially expand from smaller first diameters toany number of larger second diameters since stress is distributed moreuniformly along the cylindrical elements. This uniformity in stressdistribution reduces the tendency of stress fractures in one particularregion and allows high expansion ratios.

[0014] The different stent embodiments also allow the stents to expandto various diameters from small to large to accommodate different-sizedbody lumens, without loss of radial strength and limited contraction oflongitudinal length. The open reticulated structure of the stentsresults in a low mass device. It also enables the perfusion of bloodover a large portion of the arterial wall, which can improve the healingand repair of a damaged arterial lining.

[0015] The serpentine patterns of the cylindrical elements can havedifferent degrees of curvature of adjacent peaks and valleys tocompensate for the expansive properties of the peaks and valleys.Additionally, the degree of curvature along the peaks can be set to bedifferent in immediately adjacent areas to compensate for the expansiveproperties of the valleys adjacent to it. The more even radial expansionof this design results in stents which can be expanded to accommodatelarger diameters with minimal out of plane twisting since the highstresses arc not concentrated in any one particular region of thepattern, but are more evenly distributed among the peaks and valleys,allowing them to expand uniformly. Reducing the amount of out of planetwisting also minimizes the potential for thrombus formation.

[0016] The serpentine pattern of the individual cylindrical elements canoptionally be in phase which each other in order to reduce contractionof the stents along their length when expanded. The cylindrical elementsof the stents are plastically deformed when expanded (except with NiTialloys) so that the stents will remain in the expanded condition andtherefore they must be sufficiently rigid when expanded to prevent thecollapse thereof in use.

[0017] With stents formed from super-elastic nickel-titanium (NiTi)alloys, the expansion occurs when the stress of compression is removed.This allows the phase transformation from martensite back to austeniteto occur, and as a result the stent expands.

[0018] After the stents are expanded, some of the peaks and/or valleysmay, but not necessarily, tip outwardly and embed in the vessel wall.Thus, after expansion, the stents might not have a smooth outer wallsurface. Rather, they might have small projections which embed in thevessel wall and aid in retaining the stents in place in the vessel. Thetips projecting outwardly and strut twisting are due primarily to thestruts having a high aspect ratio. In one preferred embodiment, thestrut width is about 0.0035 inch and a thickness of about 0.0022 inch,providing an aspect ratio of 1.6. An aspect ratio of 1.0 will produceless tipping and twisting.

[0019] The elongated interconnecting members which interconnect adjacentcylindrical elements should have a transverse cross-section similar tothe transverse dimensions of the undulating components of the expandablecylindrical elements. The interconnecting members may be formed in aunitary structure with the expandable cylindrical elements formed fromthe same intermediate product, such as a tubular element, or they may beformed independently and mechanically secured between the expandablecylindrical elements.

[0020] Preferably, the number and location of the interconnectingmembers can be varied in order to develop the desired longitudinalflexibility in the stent structure both in the unexpanded as well as theexpanded condition. These properties are important to minimizealteration of the natural physiology of the body lumen into which thestent is implanted and to maintain the compliance of the body lumenwhich is internally supported by the stent. Generally, the greater thelongitudinal flexibility of the stents, the easier and the more safelythey can be delivered to the implantation site, especially where theimplantation site is on a curved section of a body lumen, such as acoronary artery or a peripheral blood vessel, and especially saphenousveins and larger vessels.

[0021] Following from the foregoing proposition is that, in general, themore interconnecting members there are between adjacent cylindricalelements of the stent, the less longitudinal flexibility there is. Moreinterconnecting members reduces flexibility, but also increases thecoverage of the vessel wall, which helps prevent tissue prolapse betweenthe stent struts. Such an approach to stent design is disclosed inco-pending patent application Ser. No. 09/008,366, filed Jan. 16, 1999,by Daniel L. Cox, entitled “Flexible Stent And Method of Use,” whoseentire contents are hereby incorporated by reference.

[0022] The present invention in particular relates to the control ofstent strength by varying the strut geometry along the length of thestent. By making the stent stronger or weaker in different regions ofthe stent, the properties can be customized to a particular application.The stent properties that could be altered include, but are not limitedto, the width of each strut, and/or the length of each cylindricalelement or ring at a constant strut thickness.

[0023] The variation of the strength of the stent affects the manner inwhich the stent expands. As expected, the wider struts tend not todeform as easily as the narrower struts during expansion, while thelonger struts within the longer cylindrical elements are better adaptedto deployment in larger diameter vessels. On the other hand, an areawith shorter cylindrical elements tends to have greater radial strengththan an area with longer cylindrical elements, given the same strutcross-sectional area.

[0024] In a preferred embodiment, the present invention is directed to alongitudinally flexible stent for implanting in a body lumen and whichis expandable from a contracted condition to an expanded condition. Thepresent invention stent preferably comprises a plurality of adjacentcylindrical elements, each cylindrical element having a circumferenceextending around a longitudinal stent axis, being substantiallyindependently expandable in the radial direction, wherein the pluralityof adjacent cylindrical elements are arranged in alignment along thelongitudinal stent axis and define a first end section, a second endsection, and a center section therebetween; each cylindrical elementhaving struts of a constant thickness formed in a generally serpentinewave pattern transverse to the longitudinal axis and containingalternating valley portions and peak portions; a plurality ofinterconnecting members extending between the adjacent cylindricalelements and connecting valley portions of adjacent cylindrical elementsto one another; and wherein the struts of at least one cylindricalelement has greater mass than the struts in other cylindrical elements.

[0025] The greater mass strut is achieved by increasing the length ofthe strut, and/or increasing the width of the strut. On the other hand,the greater mass strut is not achieved by increasing strut thickness.

[0026] In an exemplary embodiment, the present invention stent hasstruts in the cylindrical elements in the center section that have agreater mass than the struts in the cylindrical elements in the firstend section and the second end section. The greater mass is achieved byincreasing strut width and/or increasing strut length.

[0027] In another exemplary embodiment, the present invention stent hasstruts in the cylindrical elements in the center section and the secondend section that have a greater mass than the struts in the cylindricalelements in the first end section. The greater mass struts is achievedby increasing strut width and/or increasing strut length.

[0028] Still another exemplary embodiment of the present invention stentincludes struts in the first end section and the second end sectionhaving greater mass than the struts in the cylindrical elements in thecenter section. The greater mass struts is achieved by increasing strutwidth and/or increasing strut length.

[0029] Increasing or decreasing strut length in each section changes themoment arm and consequently the radial strength of that section.Increasing or decreasing strut width at a constant strut thicknesschanges the cross-sectional area of the strut and the bending moment.Hence, a wider strut has greater hoop strength and is more resistant tobending.

[0030] Other features and advantages of the present invention willbecome more apparent from the following detailed description of theinvention, when taken in conjunction with the accompanying exemplarydrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is an elevational view, partially in section, depicting thestent embodying features of the present invention mounted on a deliverycatheter disposed within a vessel.

[0032]FIG. 2 is an elevational view, partially in section, similar tothat shown in FIG. 1, wherein the stent is expanded within a vessel,pressing the lining against the vessel wall.

[0033]FIG. 3 is an elevational view, partially in section, showing theexpanded stent within the vessel after withdrawal of the deliverycatheter.

[0034]FIG. 4 is a perspective view of the stent in FIGS. 1-3 in theexpanded state.

[0035]FIG. 5 is an enlarged partial view of an alternative embodimentstent depicting the serpentine pattern with varying diameters at thepeaks and valleys.

[0036]FIG. 6 is a plan view of an alternative embodiment flattened stentof the present invention, which illustrates increased width of thestruts in a center section in between the first and second end sections.

[0037]FIG. 7 is a plan view of an alternative embodiment flattened stentof the present invention, which illustrates increased length of thecylindrical elements at the center and second end sections.

[0038]FIG. 8 is a plan view of a preferred embodiment flattened stent ofthe present invention, which illustrates increased strut width at theend sections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Prior art stent designs, such as the MultiLink Stent™manufactured by Advanced Cardiovascular Systems, Inc., Santa Clara,Calif., include a plurality of cylindrical rings that are connected bythree connecting members between adjacent cylindrical rings. Each of thecylindrical rings is formed of a repeating pattern of U-, Y-, andW-shaped members, typically having three repeating patterns forming eachcylindrical ring. A more detailed discussion of the configuration of theMultiLink Stent™ can be found in U.S. Pat. No. 5,569,295 (Lam) and U.S.Pat. No. 5,514,154 (Lau et al.), whose contents are hereby incorporatedby reference.

[0040] Beyond those prior art stents, FIG. 1 illustrates an exemplaryembodiment of stent 10 incorporating features of the present invention,which stent is mounted onto delivery catheter 11. Stent 10 generallycomprises a plurality of radially expandable cylindrical elements 12disposed generally coaxially and interconnected by interconnectingmembers 13 disposed between adjacent cylindrical elements 12. Thedelivery catheter 11 has an expandable portion or balloon 14 forexpanding stent 10 within artery 15 or other vessel. The artery 15, asshown in FIG. 1, has a dissected or detached lining 16 which hasoccluded a portion of the arterial passageway.

[0041] The delivery catheter 11 onto which stent 10 is mounted isessentially the same as a conventional balloon dilatation catheter forangioplasty procedures. The balloon 14 may be formed of suitablematerials such as polyethylene, polyethylene terephthalate, polyvinylchloride, nylon and, ionomers such as Surlyn® manufactured by thePolymer Products Division of the Du Pont Company. Other polymers mayalso be used.

[0042] In order for stent 10 to remain in place on balloon 14 duringdelivery to the site of the damage within artery 15, stent 10 iscompressed or crimped onto balloon 14. A retractable protective deliverysleeve 20 may be provided to ensure that stent 10 stays in place onballoon 14 of delivery catheter 11 and to prevent abrasion of the bodylumen by the open surface of stent 10 during delivery to the desiredarterial location. Other means for securing stent 10 onto balloon 14also may be used, such as providing collars or ridges on the ends of theworking portion, i.e., the cylindrical portion, of balloon 14. Eachradially expandable cylindrical element 12 of stent 10 may beindependently expanded. Therefore, balloon 14 may be provided with aninflated shape other than cylindrical, e.g., tapered, to facilitateimplantation of stent 10 in a variety of body lumen shapes.

[0043] In a preferred embodiment, the delivery of stent 10 isaccomplished in the following manner. Stent 10 is first mounted ontoinflatable balloon 14 on the distal extremity of delivery catheter 11.Stent 10 may be crimped down onto balloon 14 to obtain a low profile.The catheter-stent assembly can be introduced within the patient'svasculature in a conventional Seldinger technique through a slidingcatheter (not shown). Guidewire 18 is disposed through the damagedarterial section with the detached or dissected lining 16. Thecatheter-stent assembly is then advanced over guide wire 18 withinartery 15 until stent 10 is directly under detached lining 16. Balloon14 of catheter 11 is inflated or expanded, thus expanding stent 10against the inside of artery 15, which is illustrated in FIG. 2. Whilenot shown in the drawing, artery 15 is preferably expanded slightly bythe expansion of stent 10 to seat or otherwise embed stent 10 to preventmovement. Indeed, in some circumstances during the treatment of stenoticportions of an artery, the artery may have to be expanded considerablyin order to facilitate passage of blood or other fluid therethrough.

[0044] While FIGS. 1-3 depict a vessel having detached lining 16, stent10 can be used for purposes other than repairing the lining. Those otherpurposes include, for example, supporting the vessel, reducing thelikelihood of restenosis, or assisting in the attachment of a vasculargraft (not shown) when repairing an aortic abdominal aneurysm.

[0045] In general, stent 10 serves to hold open artery 15 after catheter11 is withdrawn, as illustrated in FIG. 3. Due to the formation of stent10 from an elongated tubular member, the undulating component of thecylindrical elements of stent 10 is relatively flat in a transversecross-section so that when stent 10 is expanded, cylindrical elements 12are pressed into the wall of artery 15 and as a result do not interferewith the blood flow through artery 15. Cylindrical elements 12 of stent10 that are pressed into the wall of artery 15 will eventually becovered with endothelial cell growth that further minimizes blood flowturbulence. The serpentine pattern of cylindrical sections 12 providegood tacking characteristics to prevent stent movement within theartery. Furthermore, the closely spaced cylindrical elements 12 atregular intervals provide uniform support for the wall of artery 15, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of artery 15 as illustrated in FIGS. 2 and 3.

[0046] The stresses involved during expansion from a low profile to anexpanded profile are generally evenly distributed among the variouspeaks and valleys of stent 10. As seen in the perspective view of FIG.4, each expanded cylindrical element 12 of stent 10 embodies theserpentine pattern having a plurality of peaks 36 and valleys 30 thataid in the even distribution of expansion forces. In this exemplaryembodiment, interconnecting members 13 serve to connect adjacent valleys30 of each adjacent cylindrical element 12 as described above. Thevarious peaks and valleys generally have U, Y, and W shapes, in arepeating pattern to form each cylindrical element 12.

[0047] During expansion, double-curved portions (W) 34 located in theregion of the valley where interconnecting members 13 are connected havethe most mass and accordingly are the stiffest structure duringdeformation. In contrast, peak portions (U) 36 are the least stiff, andvalley portions (Y) 30 have an intermediate stiffness.

[0048] By allocating the amount of mass to specific struts, it ispossible to create a stent having variable strength with greaterstrength at the high mass areas. Given a stent having a constantthickness in its struts, the increased mass is accomplished byincreasing the width of the strut and/or increasing the length of thestrut. The following exemplary embodiments apply this theory.

[0049]FIG. 6 is a plan view of exemplary embodiment stent 10 with thestructure flattened out into two dimensions to facilitate explanation.Stent 10 can be viewed in FIG. 6 as having three sections; namely, firstand second end sections 31A and 31B, respectively, and center section31C. As is shown, first end section 31 A has interconnecting members 13in each double-curved portion (W) 34, thereby providing maximum supportat that end of the stent. First end section 31A may optionally haveconnected thereto center section 31C as shown. But in an alternativeembodiment (not shown), center section 31C may have the same number ofinterconnecting members 13 with the same cylindrical element design asfirst end section 31A or second end section 31B. One may therefore thinkof this alternative embodiment as having center section 31C completelyomitted.

[0050] The FIG. 6 embodiment incorporates stent strut 17 in eachcylindrical element 12 at first end section 31A and second end section31B that have a narrow width as compared to the broader or wider strutwidth of stent strut 19 in center section 31B. As explained earlier,this construction can be viewed from a mass-based approach. Inparticular, at a constant strut thickness and with the presence of widerstruts 19, each cylindrical element 12 in center section 31C has agreater mass than the cylindrical elements in either the first or secondend sections 31A, 31B.

[0051] The wider struts 19 concentrate more radial or hoop strength atthe center of stent 10. This design is especially well suited to a veryfocal lesion in which a gradual transition to the normal artery isdesired.

[0052] In the exemplary embodiment of FIG. 6, each cylindrical element12 in center section 31C includes preferably three interconnectingmembers 13 to connect double curved portion (W) 34 of one cylindricalelement 12 to valley portion (Y) 30 of an adjacent cylindrical element12. In the exemplary embodiment shown in FIG. 6, interconnecting members13 within center section 42 are preferably spaced 120 degrees apart.

[0053] Each cylindrical element 12 is made up of three repeatingserpentine wave pattern sections with valley portion (Y) 30, peakportion (U) 36, and double curved portion (W) 34. Valley portion 30 andpeak portion 36 each has a generally single radius of curvature. Valleyportion 30 is connected to interconnecting member 13 and bridges to,peakportion 36. Interconnecting members 13 are preferably straight. All ofthe aforementioned structures preferably lie within center section 31C.The cylindrical clement construction is repeated for first and secondend sections 31A and 31B.

[0054] Cylindrical element 12 found within second end section 31B hasrepeating serpentine wave patterns with valley portion (Y) 30 and peakportion (U) 36, but no double curve portion (W) 34. Of course, thedifference is the omission of interconnecting members 13 in second endsection 31B to connect the cylindrical elements to yet another adjacentcylindrical element. Although contemplated but not shown, there can bemore than one cylindrical element 12 in second end section 31B.

[0055] Interconnecting members 13 may be aligned axially in every othercylindrical element 12, as shown in FIG. 6, or they may be staggereddepending on the design's bending requirements. Indeed, the presentinvention controls stent flexibility by using the number ofinterconnecting members between cylindrical elements of the stents.Generally speaking, the more interconnecting members there are betweencylindrical elements of the stent, the less longitudinal flexibilitythere is. So more interconnecting members reduces flexibility, butincreases the coverage of the vessel wall which helps prevent tissueprolapse between the stent struts.

[0056] In summary, it is contemplated that the number of cylindricalelements within first and second end sections 31A, 31B, and centersection 31C be varied as needed. The numbers and locations ofinterconnecting members 13 may be varied as needed too.

[0057] In a preferred embodiment stent 50, shown in a flattened, planview of FIG. 8, the opposite to the stent design shown in FIG. 6 couldbe applied. Here, center section 52C has narrow struts 54 and widerstruts 56 are found in first end section 52A and second end section 52B.

[0058] This preferred embodiment stent 50 could be used to give extrasupport or radial strength at the ends of the stent since the ends arenot supported by an adjacent cylindrical element. Favoring such anapproach are some existing balloon expandable stent designs that startexpanded at the ends of the stent before the center when deployed. Ifmodified in accordance with the preferred embodiment, such stents wouldexpand more consistently or homogeneously along the length of the stent.

[0059] Aside from strut widths, the overall strut pattern of stent 50 inFIG. 8 is similar to the embodiment shown in FIG. 6. Stent 50 is formedfrom individual rings or cylindrical element 58 that are linked byinterconnecting members 60. Each ring or cylindrical element 58 iscomprised of a serpentine wave pattern made up of valley portions (Y's)62, double curved portions (W's) 64, and peak portion (U's) 66.Interconnecting member 60 preferably joins valley portion (Y) 62 of onecylindrical element 58 to double curved portion (W) 64 of the adjacentcylindrical element 58. There are preferably three interconnectingmembers 60 spaced 120 degrees apart and joining each pair of adjacentcylindrical elements 58. Of course, the number and locations ofinterconnecting members 60 joining cylindrical elements 58 can bechanged as required.

[0060] In the preferred embodiment, peak portion (U) 66 has optionalstrut segment 68 shaped like a loop or bulb. Strut segment 68 may have aconstant or variable curvature to affect expansion stresses anduniformity, the details of which are explained below in connection withFIG. 5.

[0061]FIG. 7 is a plan view of alternative embodiment stent 21 flattenedinto a two-dimensional plane for the sake of illustration. Again from amass based approach, the present invention stent 21 has struts 22 in thecylindrical elements in center section 23C and second end section 23Bthat have a greater mass than struts 24 in the cylindrical elements infirst end section 23A. Looking at the exemplary embodiment of FIG. 7 abit differently, the drawing also depicts the present invention stent 21including struts 22 in the cylindrical elements in center section 23Chaving the same mass and general shape as struts 22 in the cylindricalelements in second end section 23B.

[0062] To achieve the greater mass at a constant thickness, struts ineach cylindrical element in center section 23C have a greater lengththan the struts in the cylindrical elements in first end section 23A.Alternatively, struts in each cylindrical element in center section 23Cand second end section 23B preferably have a greater length than thestruts in the cylindrical element in first end section 23A. As such,each cylindrical element in center section 23C or in second end section23B has a greater mass than a cylindrical element in first end section23A.

[0063] The embodiment of FIG. 7 uses the strut length to control theradial strength of stent 21. The struts in cylindrical elements in firstend section 23A are shorter with a shorter moment arm and thereforestronger than the long struts in center section 23C or second endsection 23B. In effect, the radial strength of stent 10 is reduced bythe increased moment arm in sections 23C and 23B.

[0064] Such a design could be used for ostial lesions in which theregion at the ostium requires more strength than the area away from theostium. The present invention stent in a balloon expandable embodimentis also well suited for use in saphenous vein grafts, because the distalend of stent 21 would open first and trap any plaque from flowingdownstream.

[0065] The arrangement of the interconnecting members 13, and therepeating serpentine wave patterns with valley portion (Y) 30, peakportion (U) 36, and double curve portion (W) 34 is similar to theembodiment shown in FIG. 6. Naturally, there can be more or fewercylindrical elements 12 than that shown in first and second end sections23A, 23B and center section 23C. The number and locations ofinterconnecting members 13 can also be varied as needed to adjustlongitudinal rigidity of stent 21.

[0066] In an alternative embodiment (not shown), the increased strutlength concept of FIG. 7 can be incorporated into the strut patternshown in FIG. 8. In such a stent, the first and second high mass endshave long struts for reduced radial or hoop strength while the centersection has shorter length struts.

[0067] As best seen in FIG. 4, because of the mass involved with thepresent invention stent designs, double curved portion (W) 34 is thestiffest structure and peak portion 36 is the least stiff structure,which account for the different stresses arising during expansion. Also,the least stiff structure, peak portion 36, is positioned between doublecurved portion 34 and valley portion 30, which are comparatively stifferstructures.

[0068] To even out the stresses, peak portion 36 in an alternativeembodiment optionally has different curvatures at regions 32 and 33, asseen in FIG. 5. Region 33 has a larger radius than region 32 and expandsmore easily. Since region 32 is adjacent the stiffer area of doublecurved portion 34, both region 32 and double curved portion 34 expandmore uniformly and more evenly distribute the expansion stresses.Further, valley portion 30 and double curved portion 34 have differentdiameters to even out the expansion forces in relation to peak portion36. Due to the unique structure as described, the shortcomings of theprior art, which include out of plane twisting of the metal, areavoided. These differing degrees of curvature along peak portion 36allow for the more even expansion of the cylindrical element 12 as awhole.

[0069] Stent 10 of FIGS. 1-6 has an expansion ratio from the crimped toexpanded configuration in the range of about, for example, 1.0 to 5.0,while maintaining structural integrity when expanded. As depicted inFIG. 4, after expansion of stent 10, portions of the various cylindricalelements 12 may turn outwardly, forming small projections 38 that embedin the vessel wall. More precisely, tip 39 of peak portion 36 tiltsoutwardly a sufficient amount upon expansion of stent 10 to embed intothe vessel wall thus helping secure implanted stent 10. Upon expansion,projections 38 create an outer wall surface on stent 10 that is notsmooth. On the other hand, while projections 38 assist in securing stent10 in the vessel-wall, they are not sharp so as to cause trauma ordamage to the vessel wall.

[0070] Tips 39 projecting outwardly and strut twisting are due primarilyto the struts having a high aspect ratio. In one preferred embodiment,the strut width is about 0.0035 inch and a thickness of about 0.0022inch, providing an aspect ratio of 1.6. An aspect ratio of 1.0 willproduce less tipping and twisting.

[0071] The dimensions of any of the foregoing exemplary embodiments canbe selected to achieve optimal expansion and strength characteristicsfor a given stent. The number of bends in each cylindrical element, asshown in FIGS. 6, 7 and 8 for example, can also be varied.

[0072] In many of the drawing figures, the present invention stent isdepicted flat, in a plan view for ease of illustration. All of theembodiments depicted herein are cylindrically-shaped stents that aregenerally formed from tubing by laser cutting as described below.

[0073] One important feature of all of the embodiments of the presentinvention is the capability of the stents to expand from a low-profilediameter to a diameter much greater than heretofore was available, whilestill maintaining structural integrity in the expanded state andremaining highly flexible. Due to the novel structures, the stents ofthe present invention each have an overall expansion ratio of about 1.0up to about 4.0 times the original diameter, or more, using certaincompositions of stainless steel. For example, a 316L stainless steelstent of the invention can be radially expanded from a diameter of 1.0unit up to a diameter of about 4.0 units, which deforms the structuralmembers beyond the elastic limit. The stents still retain structuralintegrity in the expanded state and will serve to hold open the vesselin which they are implanted. Materials other than stainless steel (316L)may afford higher or lower expansion ratios without sacrificingstructural integrity.

[0074] The stents of the present invention can be made in many ways.However, the preferred method of making the stent is to cut athin-walled tubular member, such as stainless steel tubing to removeportions of the tubing in the desired pattern for the stent, leavingrelatively untouched the portions of the metallic tubing which are toform the stent. It is preferred to cut the tubing in the desired patternby means of a machine-controlled laser.

[0075] The tubing may be made of suitable biocompatible material such asstainless steel. The stainless steel tube may be alloy-type: 316L SS,Special Chemistry per ASTM F138-92 or ASTM F139-92 grade 2. SpecialChemistry of type 316L per ASTM F138-92 or ASTM F139-92 Stainless Steelfor Surgical Implants in weight percent. Carbon (C) 0.03% max. Manganese(Mn) 2.00% max. Phosphorous (P) .025% max. Sulphur (S) 0.010% max.Silicon (Si) 0.75% max. Chromium (Cr) 17.00-19.00% Nickel (Ni)13.00-15.50% Molybdenum (Mo) 2.00-3.00% Nitrogen (N) 0.10% max. Copper(Cu) 0.50% max. Iron (Fe) Balance

[0076] The stent diameter is very small, so the tubing from which it ismade must necessarily also have a small diameter. Typically the stenthas an outer diameter on the order of about 0.06 inch in the unexpandedcondition, the same outer diameter of the tubing from which it is made,and can be expanded to an outer diameter of 0.2 inch or more. The wallthickness of the tubing is about 0.003 inch.

[0077] Generally, the tubing is put in a rotatable collet fixture of amachine-controlled apparatus for positioning the tubing relative to alaser. According to machine-encoded instructions, the tubing is thenrotated and moved longitudinally relative to the laser which is alsomachine-controlled. The laser selectively removes the material from thetubing by ablation and a pattern is cut into the tube. The tube istherefore cut into the discrete pattern of the finished stent.

[0078] The process of cutting a pattern for the stent into the tubinggenerally is automated except for loading and unloading the length oftubing. For example, a pattern can be cut in tubing using a CNC-opposingcollet fixture for axial rotation of the length of tubing, inconjunction with CNC X/Y table to move the length of tubing axiallyrelative to a machine-controlled laser as described. The entire spacebetween collets can be patterned using the CO₂, Nd or YAG laser set-upof the foregoing example. The program for control of the apparatus isdependent on the particular configuration used and the pattern to beablated in the coding.

[0079] Cutting a fine structure (0.0034 inch web width) requires minimalheat input and the ability to manipulate the tube with precision. It isalso necessary to support the tube yet not allow the stent structure todistort during the cutting operation. In order to successfully achievethe desired end results, the entire system must be configured verycarefully. The tubes are made of stainless steel with an outsidediameter of 0.060 inch to 0.100 inch and a wall thickness of 0.002 inchto 0.008 inch. These tubes are fixtured under a laser and positionedutilizing a CNC to generate a very intricate and precise pattern. Due tothe thin wall and the small geometry of the stent pattern (0.0035 inchtypical strut width), it is necessary to have very precise control ofthe laser, its power level, the focused spot size, and the precisepositioning of the laser cutting path.

[0080] In order to minimize the heat input into the stent structure,which prevents thermal distortion, uncontrolled burn out of the metal,and metallurgical damage due to excessive heat, and thereby produce asmooth debris-free cut, a Q-switched Nd/YAG, typically available fromQuantonix of Hauppauge, N.Y., that is frequency-doubled to produce agreen beam at 532 nanometers is utilized. Q-switching produces veryshort pulses (<100 nS) of high peak powers (kilowatts), low energy perpulse (<3 mJ), at high pulse rates (up to 40 kHz). The frequencydoubling of the beam from 1.06 microns to 0.532 microns allows the beamto be focused to a spot size that is two times smaller, thereforeincreasing the power density by a factor of four times. With all ofthese parameters, it is possible to make smooth, narrow cuts in thestainless steel tubes in very fine geometries without damaging thenarrow struts that make up the stent structure. Hence, the system of thepresent invention makes it possible to adjust the laser parameters tocut narrow kerf width which will minimize the heat input into thematerial.

[0081] The positioning of the tubular structure requires the use ofprecision CNC equipment such as that manufactured and sold by AnoradCorporation. In addition, a unique rotary mechanism has been providedthat allows the computer program to be written as if the pattern werebeing cut from a flat sheet. This allows both circular and linearinterpolation to be utilized in programming.

[0082] The optical system which expands the original laser beam,delivers the beam through a viewing head and focuses the beam onto thesurface of the tube, incorporates a coaxial gas jet and nozzle thathelps to remove debris from the kerf and cools the region where the beaminteracts with the material as the beam cuts and vaporizes the metal. Itis also necessary to block the beam as it cuts through the top surfaceof the tube and prevent the beam, along with the molten metal and debrisfrom the cut, from impinging on the opposite surface of the tube.

[0083] In addition to the laser and the CNC positioning equipment, theoptical delivery system includes a beam expander to increase the laserbeam diameter, a circular polarizer, typically in the form of a quarterwave plate, to eliminate polarization effects in metal cutting,provisions for a spatial filter, a binocular viewing head and focusinglens, and a coaxial gas jet that provides for the introduction of a gasstream that surrounds the focused beam and is directed along the beamaxis. The coaxial gas jet nozzle (0.018 inch I.D.) is centered aroundthe focused beam with approximately 0.010 inch between the tip of thenozzle and the tubing. The jet is pressurized with oxygen at 20 psi andis directed at the tube with the focused laser beam exiting the tip ofthe nozzle (0.018 inch dia.). The oxygen reacts with the metal to assistin the cutting process very similar to oxyacetylene cutting. The focusedlaser beam acts as an ignition source and controls the reaction of theoxygen with the metal. In this manner, it is possible to cut thematerial with a very fine kerf with precision.

[0084] In order to prevent burning by the beam and/or molten slag on thefar wall of the tube I.D., a stainless steel mandrel (approx. 0.034 inchdia.) is placed inside the tube and is allowed to roll on the bottom ofthe tube as the pattern is cut. This acts as a beam/debris barrierprotecting the far wall I.D.

[0085] Alternatively, this may be accomplished by inserting a secondtube inside the stent tube which has an opening to trap the excessenergy in the beam which is transmitted through the kerf along whichcollecting the debris that is ejected from the laser cut kerf. A vacuumor positive pressure can be placed in this shielding tube to remove thecollection of debris.

[0086] Another technique that could be utilized to remove the debrisfrom the kerf and cool the surrounding material would be to use theinner beam blocking tube as an internal gas jet. By sealing one end ofthe tube and making a small hole in the side and placing it directlyunder the focused laser beam, gas pressure could be applied creating asmall jet that would force the debris out of the laser cut kerf from theinside out. This would eliminate any debris from forming or collectingon the inside of the stent structure. It would place all the debris onthe outside. With the use of special protective coatings, the resultantdebris can be easily removed.

[0087] In most cases, the gas utilized in the jets may be reactive ornon-reactive (inert). In the case of reactive gas, oxygen or compressedair is used. Oxygen is used in this application since it offers morecontrol of the material removed and reduces the thermal effects of thematerial itself. Inert gas such as argon, helium, or nitrogen can beused to eliminate any oxidation of the cut material. The result is a cutedge with no oxidation, but there, is usually a tail of molten materialthat collects along the exit side of the gas jet that must bemechanically or chemically removed after the cutting operation.

[0088] The cutting process utilizing oxygen with the finely focusedgreen beam results in a very narrow kerf (approximately 0.0005 inch)with the molten slag re-solidifying along the cut. This traps thecut-out scrap of the pattern requiring further processing. In order toremove the slag debris from the cut allowing the scrap to be removedfrom the remaining stent pattern, it is necessary to soak the cut tubein a solution of HCL for approximately eight minutes at a temperature ofapproximately 55 degrees C. Before it is soaked, the tube is placed in abath of alcohol/water solution and ultrasonically cleaned forapproximately one minute to remove the loose debris left from thecutting operation. After soaking, the tube is ultrasonically cleaned inthe heated HCL for one to four minutes depending upon the wallthickness. To prevent cracking/breaking of the struts attached to thematerial left at the two ends of the stent pattern due to harmonicoscillations induced by the ultrasonic cleaner, a mandrel is placed downthe center of the tube during the cleaning/scrap removal process. Atcompletion of this process, the stent structures are rinsed in water.They are now ready for electropolishing.

[0089] The stents are preferably electrochemically polished in an acidicaqueous solution such as a solution of ELECTRO-GLO #300, sold by theELECTRO-GLO Co., Inc. in Chicago, Ill., which is a mixture of sulfuricacid, carboxylic acids, phosphates, corrosion inhibitors and abiodegradable surface active agent. The bath temperature is maintainedat about 110-133 degrees F. and the current density is about 0.4 toabout 1.5 amps per in². Cathode to anode area should be at least abouttwo to one. The stents may be further treated if desired, for example byapplying a biocompatible coating.

[0090] Direct laser cutting produces edges which are essentiallyperpendicular to the axis of the laser cutting beam, in contrast withchemical etching and the like which produce pattern edges which areangled. Hence, the laser cutting process of the present inventionessentially provides stent cross-sections, from cut-to-cut, which aresquare or rectangular, rather than trapezoidal. The resulting stentstructure provides superior performance.

[0091] The stent tubing may be made of suitable biocompatible materialsuch as stainless steel, titanium, tantalum, super-elastic(nickel-titanium) NiTi alloys and even high strength thermoplasticpolymers. The stent diameters are very small, so the tubing from whichit is made must necessarily also have a small diameter. For PCTAapplications, typically the stent has an outer diameter on the order ofabout 1.65 mm (0.065 inches) in the unexpanded condition, the same outerdiameter of the hypotubing from which it is made, and can be expanded toan outer diameter of 5.08 mm (0.2 inches) or more. The wall thickness ofthe tubing is about 0.076 mm (0.003 inches). For stents implanted inother body lumens, such as PTA applications, the dimensions of thetubing are correspondingly larger. While it is preferred that the stentsbe made from laser cut tubing, those skilled in the art will realizethat the stent can be laser cut from a flat sheet and then rolled up ina cylindrical configuration with the longitudinal edges welded to form acylindrical member.

[0092] In the instance when the stents are made from plastic, theimplanted stent may have to be heated within the arterial site where thestents are expanded to facilitate the expansion of the stent. Onceexpanded, it would then be cooled to retain its expanded state. Thestent may be conveniently heated by heating the fluid within the balloonor the balloon itself directly by a known method.

[0093] The stents may also be made of materials such as super-elastic(sometimes called pseudo-elastic) nickel-titanium (NiTi) alloys. In thiscase the stent would be formed full size but deformed (e.g. compressed)to a smaller diameter onto the balloon of the delivery catheter tofacilitate intraluminal delivery to a desired intraluminal site. Thestress induced by the deformation transforms the stent from an austenitephase to a martensite phase, and upon release of the force when thestent reaches the desired intraluminal location, allows the stent toexpand due to the transformation back to the more stable austenitephase. Further details of how NiTi super-elastic alloys operate can befound in U.S. Pat. Nos. 4,665,906 (Jervis) and 5,067,957 (Jervis),incorporated herein by reference in their entirety.

[0094] While the invention has been illustrated and described herein interms of its use as intravascular stents, it will be apparent to thoseskilled in the art that the stents can be used in other instances in allvessels in the body. Since the stents of the present invention have thenovel feature of expanding to very large diameters while retaining theirstructural integrity, they are particularly well suited for implantationin almost any vessel where such devices are used. This feature, coupledwith limited longitudinal contraction of the stent when they areradially expanded, provide a highly desirable support member for allvessels in the body. Other modifications and improvements may be madewithout departing from the scope of the invention.

What is claimed is:
 1. A longitudinally flexible stent for implanting ina body lumen and expandable from a contracted condition to an expandedcondition, comprising: a plurality of adjacent cylindrical elements,each cylindrical element having a circumference extending around alongitudinal stent axis, being substantially independently expandable inthe radial direction, wherein the plurality of adjacent cylindricalelements are arranged in alignment along the longitudinal stent axis anddefine a first end section, a second end section, and a center sectiontherebetween; each cylindrical element having constant thickness strutsformed in a generally serpentine wave pattern transverse to thelongitudinal axis and containing alternating valley portions and peakportions; a plurality of interconnecting members extending between theadjacent cylindrical elements and connecting the adjacent cylindricalelements to one another; and wherein the struts in at least onecylindrical element have a greater mass than the struts of the othercylindrical elements.
 2. The stent of claim 1, wherein the struts of thecylindrical elements in the center section have a greater mass than thestruts of the cylindrical elements in the first end section and thesecond end section.
 3. The stent of claim 1, wherein the struts of thecylindrical elements in the first end section and the second end sectionhave a greater mass than the struts of the cylindrical elements in thecenter section.
 4. The stent of claim 2, wherein the struts havinggreater mass have greater strut width.
 5. The stent of claim 2, whereinthe struts having greater mass have greater strut length.
 6. The stentof claim 3, wherein the struts having greater mass have greater strutwidth.
 7. The stent of claim 3, wherein the struts having greater masshave greater strut length.
 8. The stent of claim 1, wherein thecylindrical elements in the center section and the second end sectionhave a greater length than the cylindrical elements in the first endsection.
 9. The stent of claim 1, wherein the peak portions haveirregular radii of curvature so that upon expansion, the peak portionsuniformly and evenly expand.
 10. The stent of claim 1, wherein the stentis formed from a flat piece of material.
 11. The stent of claim 1,wherein the stent is formed of a biocompatible material selected fromthe group consisting of stainless steel, tungsten, tantalum,super-elastic nickel-titanium alloys, or thermoplastic polymers.
 12. Thestent of claim 1, wherein the stent has a radial expansion ratio ofabout 1.0 in the contracted condition up to about 4.0 in the expandedcondition.
 13. The stent of claim 1, wherein the stent is formed from asingle piece of tubing.
 14. A longitudinally flexible stent forimplanting in a body lumen and expandable from a contracted condition toan expanded condition, comprising: a plurality of adjacent cylindricalelements, each cylindrical element having a circumference extendingaround a longitudinal stent axis, being substantially independentlyexpandable in the radial direction, wherein the plurality of adjacentcylindrical elements are arranged in alignment along the longitudinalstent axis and define a first end section, a second end section, and acenter section therebetween; each cylindrical element having strutsformed in a generally serpentine wave pattern transverse to thelongitudinal axis and containing alternating valley portions and peakportions; a plurality of interconnecting members extending between theadjacent cylindrical elements and connecting the adjacent cylindricalelements to one another; and wherein the struts of each cylindricalelement in the center section have a greater mass than the struts ofeach cylindrical element in the first end section.
 15. The stent ofclaim 14, wherein the struts of each cylindrical element in the centersection have a greater mass than the struts of each cylindrical elementin the first end section and the second end section.
 16. The stent ofclaim 14, wherein the struts of each cylindrical element in the centersection and the second end section have a greater mass than the strutsof each cylindrical element in the first end section.
 17. The stent ofclaim 14, wherein the struts of each cylindrical element in the centersection have the same mass as the struts of each cylindrical element inthe second end section.
 18. The stent of claim 14, wherein thecylindrical elements cooperate to define a generally smooth cylindricalsurface and wherein the peak portions form projecting edges whichproject outwardly from the cylindrical surface upon expansion.
 19. Thestent of claim 14, wherein the struts of each cylindrical element in thecenter section have a greater strut width than the struts of eachcylindrical element in the first end section and the second end section.20. The stent of claim 14, wherein each cylindrical element in thecenter section has a greater length than each cylindrical element in thefirst end section.
 21. The stent of claim 14, wherein each cylindricalelement in the center section and the second end section have a greaterlength than each cylindrical element in the first end section.
 22. Amethod for constructing a longitudinally flexible stent for implantingin a body lumen and expandable from a contracted condition to anexpanded condition, the method comprising the steps of: providing aplurality of adjacent cylindrical elements, each cylindrical elementhaving a circumference extending around a longitudinal stent axis, beingsubstantially independently expandable in the radial direction;arranging the plurality of adjacent cylindrical elements in alignmentalong the longitudinal stent axis to include a first end section, asecond end section, and a center section therebetween; forming struts ineach cylindrical element in a generally serpentine wave patterntransverse to the longitudinal axis and containing alternating valleyportions and peak portions; providing a plurality of interconnectingmembers extending between the adjacent cylindrical elements andconnecting the adjacent cylindrical elements to one another; andproviding struts of the cylindrical elements in the center section thathave a greater mass than the struts of the cylindrical elements in thefirst end section.
 23. The method of claim 22, wherein the methodfurther comprises providing struts of the cylindrical elements in thecenter section having a greater mass than the struts of the cylindricalelements in the first end section and the second end section.
 24. Themethod of claim 22, wherein the method further comprises the step ofproviding struts of the cylindrical elements in the center section andthe second end section having a greater mass than the struts of thecylindrical elements in the first end section.
 25. The method of claim22, wherein the method further comprises the step of providing struts ofthe cylindrical elements in the center section having the same mass asthe struts of the cylindrical elements in the second end section. 26.The method of claim 22, wherein the method further comprises the step ofproviding struts of the cylindrical elements in the center sectionhaving a greater strut width than the struts of the cylindrical elementsin the first end section.
 27. The method of claim 22, wherein the methodfurther comprises the step of providing struts of the cylindricalelements in the center section have a greater strut width than thestruts of the cylindrical elements in the first end section and thesecond end section.
 28. The method of claim 22, wherein the methodfurther comprises the step of providing each cylindrical element in thecenter section having a greater length than the cylindrical elements inthe first end section.
 29. The method of claim 22, wherein the methodfurther comprises the step of providing cylindrical elements in thecenter section and the second end section having a greater length thanthe cylindrical elements in the first end section.
 30. A longitudinallyflexible stent for implanting in a body lumen and expandable from acontracted condition to an expanded condition, comprising: a pluralityof adjacent cylindrical elements, each cylindrical element having acircumference extending around a longitudinal stent axis, beingsubstantially independently expandable in the radial direction, whereinthe plurality of adjacent cylindrical elements are arranged in alignmentalong the longitudinal stent axis and define a first end section, asecond end section, and a center section therebetween; each cylindricalelement having struts formed in a generally serpentine wave patterntransverse to the longitudinal axis and containing alternating valleyportions and peak portions; a plurality of interconnecting membersextending between the adjacent cylindrical elements and connecting theadjacent cylindrical elements to one another; and wherein the struts ofthe cylindrical elements in the first section have a greater mass thanthe struts of the cylindrical elements in the center section.
 31. Thestent of claim 30, wherein the struts of the cylindrical elements in thesecond end section have a greater mass than the struts of thecylindrical elements in the center section.
 32. The stent of claim 31,wherein the struts of the cylindrical elements have a constantthickness.
 33. The stent of claim 30, wherein the struts in the firstend section have a longer strut length than the struts in thecylindrical elements in the center section.
 34. The stent of claim 30,wherein the struts in the first end section have a greater strut widththan the struts in the cylindrical elements in the center section. 35.The stent of claim 30, wherein the struts in the first end section andthe second end section have greater strut widths than the struts in thecylindrical elements in the center section.
 36. A method forconstructing a longitudinally flexible stent for implanting in a bodylumen and expandable from a contracted condition to an expandedcondition, the method comprising the steps of: providing a plurality ofadjacent cylindrical elements, each cylindrical element having acircumference extending around a longitudinal stent axis, beingsubstantially independently expandable in the radial direction;arranging the plurality of adjacent cylindrical elements in alignmentalong the longitudinal stent axis to include a first end section, asecond end section, and a center section therebetween; forming struts ineach cylindrical element in a generally serpentine wave patterntransverse to the longitudinal axis and containing alternating valleyportions and peak portions; providing a plurality of interconnectingmembers extending between the adjacent cylindrical elements andconnecting the adjacent cylindrical elements to one another; andproviding struts of the cylindrical elements in the first end sectionthat have a greater mass than the struts of the cylindrical elements inthe center section.
 37. The method of claim 36, wherein the methodfurther comprises providing struts of the cylindrical elements in thefirst end section having a greater strut length than the struts of thecylindrical elements in the center section.
 38. The method of claim 36,wherein the method further comprises providing struts of the cylindricalelements in the first end section having a greater strut width than thestruts of the cylindrical elements in the center section.
 39. The methodof claim 36, wherein the method further comprises the step of providingstruts of constant thickness in the cylindrical elements.
 40. The methodof claim 36, wherein the method further comprises providing struts ofthe cylindrical elements in the first end section and the second endsection having a greater strut width than the struts of the cylindricalelements in the center section.
 41. The method of claim 36, wherein themethod further comprises providing struts of the cylindrical elements inthe first end section and the second end section having a greater strutlength than the struts of the cylindrical elements in the centersection.